19 October 2009

Looking at the genitals of naked mole rats

Since Heterocephalus glaber are naked mole rats, they shouldn’t mind anyone looking at their sex organs, right? I mean, they’re naked, so it’s not like they’re ashamed or anything.

Seney and colleagues noticed that male and female naked mole rats had very similar looking sex organs. Naked mole rats happen to be famous for having a social system that is very much like social insects, with normally a single reproductive adult female and non-reproductive workers. They wondered if this unsual similarity might be related to the social structure of the mole rats.

Rufus notwithstanding (below), naked mole rats are obliged to live in social colonies, and will die if alone, so they couldn’t really do an experiment. Instead, they went looking for another species that showed some similar social features, and found the Damaraland mole-rat (Fukomys damarensis). It also lives in colonies, but the colonies are smaller, meaning more individuals, on average, reproduce. Seney and colleague predicted that the Damaraland mole-rat should have genitals that were more similar in males and females that other, non-social mole rats.

In short, that’s pretty much what they found. Out of six features they authors examined, the Damaraland mole-rat resembled the non-social mole rats on three measures, the naked mole rats on one, and sat in the middle on two.

Interpreting these results hinges upon the relatedness of these species. The authors note that sociality has either evolved more than once, or been lost more than once within the family. The authors argue that the naked and Damaraland mole rats “seem” to have evolved sociality separately, but the information presented in this paper don’t do a great job of convincing here.

The authors also note that the naked mole rat situation seems to be the reverse situation to hyenas. Hyenas also have very similar genitals, but this seems to be because the females’ genitals have been masculinized to resemble the males’. Here, the males’ genitals seem to be feminized to resemble the females’. It’s not at all clear, however, what the relevant selection pressure, or lack thereof, is that is creating similar genitals. It’s slightly surprising that even the “king” males in a naked mole rat colony, that actually do breed, stills has genitals that resemble the females.